The Application of Fulvic Acid Can Enhance the Performance of Rice Seedlings Under Low-Nitrogen Stress

Fulvic acid's potential to enhance plant growth has been recognized, but its effects on plant growth and nutrient uptake under nutrient stress remain unclear. This experiment investigated the effects of fulvic acid at concentrations of 0 mg L^-1 (T1), 30 mg L^-1 (T2), 60 mg L^-1 (T3), 90 mg L^-1 (T4), 120 mg L^-1 (T5), and 150 mg L^-1 (T6) on the growth performance of two rice varieties-Jikedao 654 (J 654) and Jiyang 100 (J 100)-under low-nitrogen stress in a hydroponic system. The effects of different fulvic acid application rates on the growth and photosynthetic characteristics, the key enzymes of nitrogen metabolism, antioxidant properties, and the osmotic adjustment substances of rice under low-nitrogen stress were evaluated. The results indicated that the addition of an appropriate concentration of fulvic acid could enhance the growth performance of J 654 and J 100 under low-nitrogen stress. Compared to T1 treatment, the total dry weight and nitrogen accumulation of rice showed greater increases in response to T3 and T4 treatments. The photosynthetic pigment content increased, photosynthesis was enhanced, and the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), and transpiration rate (Tr) were improved. The activities of key enzymes in nitrogen metabolism, including nitrate reductase (NR), glutamine synthetase (GS), glutamate oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase activity (GPT), were enhanced, thereby improving the capacity for nitrogen uptake and assimilation. The addition of fulvic acid also enhanced the antioxidant capacity, increased the superoxide dismutase (SOD), peroxide (POD) and catalase (CAT) activity and decreased the toxic effects of ROS, the production rate of O₂^-, and the hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) content. The low-nitrogen stress was alleviated, thereby reducing the proline and soluble sugars content. Overall, it was demonstrated that adding an appropriate concentration (60-90 mg L^-1) of fulvic acid under low-nitrogen stress has a positive impact on the growth and development of rice. Our findings provide a theoretical basis for the application of fulvic acid in alleviating low-nitrogen stress in rice.